Animate many objects each object has many animations
What you want is a Modular Rig that can support all your weapons, this has trade-offs for example, you will have to make all weapons work the same way, you cannot rig a knife and expect it to work the same way like a rifle, but you can rig a pistol using a rifle rig, that's for a simple rig. But, if your rig is good enough, then it can adapt to some extent anything you throw at it, but that usually will require a average amount of knowledge in constraints and rigging.

That being said, if you are a beginner in blender, this is an advanced project and I am afraid you will not find a easy solution. I can point you to a great rigging tutorial with great techniques, but it's not for beginners, so keep that in mind. https://www.youtube.com/watch?v=lqiA9E9dHnk

Hope it helps,